A cyclotron is a charged particle accelerator wherein particles are guided along a quasi-circular or spiral path around an axis by a magnetic field, and accelerated each time they pass an accelerating gap along their path. The particles may be produced in the central region of the cyclotron by an ion source. After a number of turns along the quasi-circular or circular path, the particle beam is usually extracted from the cyclotron for directing the beam onto a target e.g. for producing a radioisotope or other purposes. Several methods are known for extracting the particles. One such method is the stripping method: the ions produced by the source are negative ions. At the extraction region, they pass through a thin foil of material which strips off the weakly bound electrons from the ions. The emerging positively charged particles are now deflected in the opposite direction by the magnetic field and directed outside of the cyclotron. This method is efficient, but has the drawback that negative ions may be produced and accelerated. These negative ions are fragile and can lose an electron in the course of acceleration, and then produce unwanted isotopes and activate the cyclotron. In addition, strippers are very thin foils and may be destroyed by the beam, and may require maintenance.
Another method is to arrange at the periphery of the cyclotron a pair of electrodes producing an electrical field pulling the charged particles out of the cyclotron. The inner electrode, called septum, may be placed in the distance between two successive turns of the cyclotron. However, this method works when there is a sufficient separation between two successive turns at the extraction region. When the separation distance between turns is small, or the beam has some radial extension, the septum receives part of the particles and becomes activated.
Another extraction method, known as auto-extraction, is disclosed in WO0105199A1. This method is applicable to cyclotrons having azimuthally varying fields also called sector-focused cyclotrons. In these cyclotrons, the poles are divided in sectors, where the vertical gap is small, called hills, separated by sectors where the vertical gap is large, called valleys. The auto-extraction is obtained by providing a hill sector having a significantly larger radial extension. A groove or plateau, which follows the shape of the particle path, is provided in the extended part of this extended sector. The resulting local dip in the magnetic field forces the beam to exit the cyclotron. In order to ensure that coherent oscillations are such that the beam enters the groove or plateau, it is necessary to locate a first pair of harmonic coils producing opposite vertical field components at 180° from each other, and a second pair of harmonic coils also producing opposite vertical field components at 180° from each other, at 90° from the first pair. Such a pair of harmonic coils have the effect of displacing laterally the center of the quasi circular beam path. The currents in the first and second harmonic coils may be tuned in such a way that the beam enters the groove or plateau.
As an alternative to the extraction methods discussed above, the accelerated beam may be used without extraction, by locating a target receiving the beam inside the cyclotron, at the periphery of the cyclotron.
All the methods described above are adapted for cyclotrons having a single source and a single particle beam. A cyclotron having two ion sources is described in EP2196073A1. As discussed in said document, there is a need for a cyclotron having two ion sources. The ion sources described in this document may be used alternatively or simultaneously, thereby increasing either the uptime and reliability or the productivity of the cyclotron. When used alternatively, the second ion source may be used when a defect occurs in the first ion source. The second source may take over at once, with reduced downtime, and reduced need for the maintenance personnel to enter the shielded room of the cyclotron. When used simultaneously, the beam current produced may be twice as high, reaching two distinct targets. The designs described in this document are obtained by adapting a known design, discussed at paragraph 29 and represented at FIG. 4 of this document, and where one of the sources produces protons and the other source produces deuterons. These two different sources are replaced by two identical sources. As can be seen on said FIG. 4, the ions produced by one source hit the backside of the other ion source and the beam is lost during the first turn. Two solutions to this problem are proposed:                (i) ion sources having a special design, including a notch, have been designed.        (ii) the two ions sources are shifted towards the center bringing them in the closest geometry that is technical possible in view of the dimensions of the ion sources.Although these two solutions allow the design orbit emitted from one source to pass radially outwards of the other source at the first half-turn, these solutions have disadvantages:        (i) the sources having a notch are more complex to produce and more fragile        (ii) when shifting the sources towards the center, the gap between the sources and the Dee electrode is not optimal. E.g., the electric field lines may not be optimal for accelerating particles, or for a given voltage between source and Dee, the electric field may be very high and lead to breakdowns.Moreover, said document does not address the problem of extraction of the two beams. The auto-extraction discussed above will not work with this twin source design, because the harmonic coils will have the desired effect for one of the beams, but will not be able to direct both beams simultaneously.        
The present disclosure aims to provide a cyclotron which overcomes the problem of the prior art. In particular, it is an object of the present disclosure to provide a twin ion source cyclotron capable of producing high beam currents, low beam losses, with robust ion sources.